EPCO-16. ONCOHISTONE INTERACTOME PROFILING UNCOVERS MECHANISMS OF CHROMATIN DISRUPTION AND IDENTIFIES POTENTIAL THERAPEUTIC TARGETS IN PEDIATRIC HIGH-GRADE GLIOMA

Mutations in histone H3 at amino acids 27 (H3K27M) and 34 (H3G34R) occur with high-frequency in pediatric high-grade glioma. H3K27M mutations have been shown to lead to global disruption of H3K27me3 through dominant negative PRC2 inhibition with accompanying gains in H3K36me3, while H3G34R mutations lead to local losses of H3K36me3 through inhibition of SETD2. However, the mechanism of action of these mutants on the broader landscape of chromatin-associated proteins remains unknown. Importantly, proteins with differential associations with oncohistones could be targeted therapeutically. Here we profiled the interactomes of the H3.1K27M, H3.3K27M and H3.3G34R oncohistones using BioID to gain an unbiased measure of their interaction landscapes. Among the differential interactors all 3 mutants lost interaction with H3K9 methyltransferases, while H3G34R also had reduced interaction with DNA methyltransferases accompanied by genome-wide DNA hypomethylation. In contrast, H3K27M mutants had increased association with transcription factors, consistent with the activation of transcription induced by the global loss of H3K27me3. H3K9me3 was reduced in H3K27M-containing nucleosomes, and cis-H3K9 methylation was required for H3K27M to exert its effect on global H3K27me3. Depletion of H3K9 methyltransferases with shRNA or treatment with H3K9 methyltransferase inhibitors was lethal to H3.1K27M, H3.3K27M and H3.3G34R mutant pHGG cell lines, underscoring the importance of H3K9 methylation for oncohistone-mutant gliomas and suggesting it could make an attractive therapeutic target.

Transitioning to Molecular Diagnostics in Pediatric High Grade Glioma: Experiences with the 2016 WHO Classification of CNS Tumors

Background Pediatric neuro-oncology was profoundly changed in the wake of the 2016 revision of the WHO Classification of Tumors of the Central Nervous System. Practitioners were challenged to quickly adapt to a system of tumor classification redefined by molecular diagnostics. Methods We designed a 22 question survey studying the impact of the revised WHO classification on pediatric high-grade glioma. The survey collected basic demographics, general attitudes, issues encountered, and opinions on pediatric subtypes. Participant answers were analyzed along socioeconomic lines utilizing the human development index (HDI) of the United Nations and membership in the group of seven (G7) world economic forum. Results 465 participants from 53 countries were included, 187 pediatric neurooncologists (40%), 160 neuropathologists (34%), and 118 other experts (26%). When asked about pediatric high grade glioma entities, participants from very high development countries preferred treating a patient based on genetic findings. Participants from high and medium development countries indicated using traditional histology and tumor location as mainstays for therapeutic decisions. Non-G7 countries tended to regard the introduction of molecularly characterized tumor entities as a problem for daily routine due to lack of resources. Conclusions Our findings demonstrate an overall greater reliance and favorability to molecular diagnostics among very high development countries. A disparity in resources and access to molecular diagnostics has left some centers unable to classify pediatric high-grade glioma per the WHO classification. The forthcoming edition should strain to abate disparities in molecular diagnostic availability and work towards universal adaptation.

The Impact of Radiation Therapy Variables on Pediatric High-Grade Glioma Outcomes: A National Cancer Database Analysis

Purpose Pediatric high-grade glioma (HGG) is a devastating disease with a poor prognosis. The purpose of this analysis is to evaluate the impact of Radiation Therapy (RT) variables on outcomes of pediatric HGG patients in the National Cancer Database (NCDB). Methods The NCDB was used to select patients age < 22 with histologically proven WHO Grade III and IV gliomas treated with ≥ 50Gy and < 76Gy RT between 2004 and 2013. RT variables including RT dose, timing between diagnosis and RT initiation (< 4 weeks, 4–6 weeks, or > 6 weeks), and RT modality were analyzed along with baseline demographic, tumor and treatment variables to assess the impact on overall survival in univariate and multivariable analyses. Results 498 pediatric HGG patients were included. Histologies included glioblastoma (30%), astrocytoma (55%), oligogendroglioma (5%) and gliomas not otherwise specific (10%). The median RT dose was 59.4 Gy (SD 2.9 Gy) starting a median of 4.4 weeks from diagnosis (SD 2.5 weeks). Median follow-up was 19.6 months with 1- and 3-year OS of 78.4% and 40.4%, respectively. On Multivariable analysis, female gender, older age, and private insurance remained independently associated with lower rate of overall death. Radiation initiation ≤ 4 weeks from diagnosis, and glioblastoma histology were significantly associated with higher rate of overall death. There was no relationship between radiation dose or whether radiation was delivered with proton or photon therapy and overall survival. Conclusions Outcomes for pediatric HGG are poor. Early initiation of RT within 4 weeks from diagnosis was negative associated with overall survival and may be related to unknown prognostic factors.